As feature sizes have been constantly shrinked, conventional silicon-based MOS devices have been confronted with numerous challenges and limitations, such as mobility degradation. DIBL, hot carrier effects, and NBTI. Among them, the problem of mobility degradation may affect improvement for operating speed of the integrated circuit. Hence, it is in urgent need of seeking a new material or a new device to overcome this problem. Since germanium has a larger electron and hole mobility than that of silicon and a lower impurity activation temperature, and the fabrication process of a germanium channel device is compatible with a conventional CMOS process, germanium has obtained extensive attentions. Currently, however, the fabrication technology of the germanium-based MOS device has not been matured yet, and the performance of the device is not quite stable. One of reasons limiting development of the germanium-based MOS device is that an interface characteristic between gate dielectric and substrate of the germanium-based MOS device is not quite satisfactory. Therefore, it is an important process step to remove a natural oxide layer on the surface of germanium-based substrate and passivate the surface of the germanium-based substrate so as to alleviate the natural oxidation once more.